I watched a few days ago a movie, a boxer killed someone with his fists. The judge said "in California, your education and skills are accepted as deadly weapon"...(the guy went to the prison)...as I know, in my country, your level of MA skill is also taken into account...

Definition: Deadly Force is actions against another which have a high liklihood of inflicting serious and lasting bodily harm.

You are justified in using Deadly Force any time you are presented with a threat of that magnitude against you or another person. Your perception of this threat must be considered "reasonable" by a jury of your peers.

You may only use Force to stop the threat. Once the threat is removed, you must stop. You may not use Force to punish, you may not use Force to protect your property. Yo u may only use force to protect yourself or another from a reasonably perceived threat. The amount of Force must be "reasonable" to stop the threat.

Deciding whther the threat justifies Force is governed (for civilians) by the following factors:
Ability: the threat must come from somone who can genuinely hurt you; this is where traiing comes into the equation. If you are a highly trained fighter, the Law will assume that a given threat must be greater for you to really be threatened than it would be for an untrained person; factors such as stature and strength also come to play as well as numvers of attackers vs numbers of defeneders

Opportunity: does a given person actually have the opportunity to actually harm you? I might know a guy would like to punch me out but he isn't currently around. I do not have the right to go find him and beat him up as a preventive measure.

Jeopardy: Jeopardy is closely related to opportunity. I need to be in imminent jeopardy to justify force. The guy who wants to beat me up has enetered the same room I am in. He now has the opportunity but he hasn't made a move to threaten me; therefore I am not in jeopardy.

Preclusion: Was there something I could have done to avoid the situation or escape from the threat? As a civilian you are expected to use preclusion to avoid using force. For instance in a bar, you might have a situation in which abilty, opportunity, and jeopardy are all present. If you use force in this situation and it is later determined that you could have simply left the bar to avoid the threat, then you are in trouble.

A guy with a knife is by definition Deadly Force. You can use whatever means a "reasonable" person would use to end that threat. You can shoot him (assuming that you are allowed to have a gun and even if not, the charge would be violation of the gun law, not excessive force). You can hit him in the head with a brick, whatever it takes. But the instant he drops the knife you are at a different level of force. As a trained martial artist you would be ok using deadly force against an armed attacker but might not be against an unarmed attacker. It would be dependent on other factors such as his perceived abilty to fight, his size and strength, any indicators as to his intentions (I'm going to kill you is a good one), etc.

This is precisely why Aikido is good to know as you have the ability to de-escalate as appropriate. You can move from deadly force locks and throws to less than deadly control techniques as needed. Many of these techniques don't "look bad" when seen by an untrained person so it is easy to articulate why you used a given technqiue at a certain level of threat.